Analysis of aircraft and satellite measurements from the …fizz.phys.dal.ca/~atmos/publications/vanDonkelaar_2008... · 2008. 6. 17. · 3000 A. van Donkelaar et al.: Long-range

Atmos. Chem. Phys., 8, 2999–3014, 2008www.atmos-chem-phys.net/8/2999/2008/© Author(s) 2008. This work is distributed underthe Creative Commons Attribution 3.0 License.

AtmosphericChemistry

and Physics

Analysis of aircraft and satellite measurements from theIntercontinental Chemical Transport Experiment (INTEX-B) toquantify long-range transport of East Asian sulfur to Canada

A. van Donkelaar1, R. V. Martin 1,2, W. R. Leaitch3, A. M. Macdonald3, T. W. Walker 1,4, D. G. Streets5, Q. Zhang5,E. J. Dunlea6, J. L. Jimenez6, J. E. Dibb7, L. G. Huey8, R. Weber8, and M. O. Andreae9

1Dept. of Physics and Atmospheric Science, Dalhousie University, Canada2Harvard-Smithsonian Center for Astrophysics, USA3Science and Technology Branch, Environment Canada, Canada4Dept. of Physics, University of Toronto, Canada5Decision and Information Sciences Division, Argonne National Laboratory, USA6Department of Chemistry and Biochemistry, and Cooperative Institute for Research in the Environmental Sciences (CIRES),University of Colorado, USA7Climate Change Research Center/EOS, University of New Hampshire, USA8School of Earth and Atmospheric Sciences, Georgia Institute of Technology, USA9Biogeochemistry Department, Max Planck Institute for Chemistry, Germany

Received: 11 December 2007 – Published in Atmos. Chem. Phys. Discuss.: 27 February 2008Revised: 19 May 2008 – Accepted: 20 May 2008 – Published: 17 June 2008

Abstract. We interpret a suite of satellite, aircraft, andground-based measurements over the North Pacific Oceanand western North America during April–May 2006 aspart of the Intercontinental Chemical Transport ExperimentPhase B (INTEX-B) campaign to understand the implica-tions of long-range transport of East Asian emissions toNorth America. The Canadian component of INTEX-Bincluded 33 vertical profiles from a Cessna 207 aircraftequipped with an aerosol mass spectrometer. Long-rangetransport of organic aerosols was insignificant, contrary toexpectations. Measured sulfate plumes in the free tropo-sphere over British Columbia exceeded 2µg/m3. We up-date the global anthropogenic emission inventory in a chem-ical transport model (GEOS-Chem) and use it to interpret theobservations. Aerosol Optical Depth (AOD) retrieved fromtwo satellite instruments (MISR and MODIS) for 2000–2006are analyzed with GEOS-Chem to estimate an annual growthin Chinese sulfur emissions of 6.2% and 9.6%, respectively.Analysis of aircraft sulfate measurements from the NASADC-8 over the central Pacific, the NSF C-130 over the eastPacific and the Cessna over British Columbia indicates most

Correspondence to:A. van Donkelaar([email protected])

Asian sulfate over the ocean is in the lower free troposphere(800–600 hPa), with a decrease in pressure toward land dueto orographic effects. We calculate that 56% of the mea-sured sulfate between 500–900 hPa over British Columbiais due to East Asian sources. We find evidence of a 72–85% increase in the relative contribution of East Asian sul-fate to the total burden in spring off the northwest coast ofthe United States since 1985. Campaign-average simulationsindicate anthropogenic East Asian sulfur emissions increasemean springtime sulfate in Western Canada at the surface by0.31µg/m3(∼30%) and account for 50% of the overall re-gional sulfate burden between 1 and 5 km. Mean measureddaily surface sulfate concentrations taken in the Vancouverarea increase by 0.32µg/m3 per 10% increase in the simu-lated fraction of Asian sulfate, and suggest current East Asianemissions episodically degrade local air quality by more than1.5µg/m3.

1 Introduction

The transport of Asian emissions to North America has beenwell documented (e.g. Parrish et al., 1992; Jaffe et al., 1999;Bertschi et al., 2004; Liang et al., 2004; Park et al., 2004).Andreae et al. (1988) measured sulfate (SO=4 ) concentrations

Published by Copernicus Publications on behalf of the European Geosciences Union.

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3000 A. van Donkelaar et al.: Long-range transport of East Asian sulfur to Canada

Fig. 1. Spatial domain of observations used to characterize AsianSO2 emissions and their impact. The domains and flightpaths of theDC-8, C-130 and Cessna aircraft are shown in blue, red and green,respectively. The domain of the MODIS and MISR satellite obser-vations used to estimate emissions is shown in yellow. Figure 2shows a detailed plot of the Cessna flight tracks.

off the northwest coast of the United States in May 1985,and attributed enhancements in the free troposphere to Asiansources. Asian emissions of sulfur oxides (SOx=SO2+SO=4 )are dominated by SO2 and have grown substantially over thelast two decades (Streets and Waldhoff, 2000). They increas-ingly impact North America, affecting both regional air qual-ity (Park et al., 2004; Heald et al., 2006) and climate (Liu etal., 2008). Additional analysis of in-situ and remote-sensedobservations are needed to quantify this long-range transportand its implications.

A growing body of evidence exists for long-range trans-port to Canada. During the Polar Sunrise Experiment in 1992at Alert concentrations of SOx were well correlated withlong-range transport of fine anthropogenic aerosol (Barrie etal., 1994; Sirois and Barrie, 1999) analyzed aerosol composi-tion between 1980 and 1995 to infer the presence of EurasianSO=4 in the Canadian Arctic. Asian pesticides have been ob-served in the Yukon Territory as a result of transpacific flow(Bailey et al., 2000). The influence of long-range transportto Canada is not limited to remote regions, and is especiallyrelevant in populated areas. Asian pesticides have been trans-ported to the Fraser Valley, British Columbia (Harner et al.,2005). Chinese dust has been observed in British Columbia’sLower Fraser Valley (McKendry et al., 2001) and can belinked to SO=4 transport through the uptake of sulfur dioxide(SO2) on dust (Jordan et al., 2003; Song et al., 2007). Dusttransport to western Canada has also been observed from asfar as the Sahara Desert (McKendry et al., 2007). Althoughaerosol in the Asian boundary layer may be readily scav-enged near its source by wet deposition, SO2 emissions canescape to the free troposphere prior to SO=4 conversion andbe subsequently transported across the Pacific Ocean (Brocket al., 2004; Dunlea et al., 20081). Elevated aerosol concen-trations, attributed to East Asian combustion sources, have

1Dunlea, E., DeCarlo, P. F., Kimmel, J. R., Aiken, A. C., Peltier,R., Weber, R., Tomlison, J., Collins, D., Shinozuka, Y., Howell,S., Clarke, A., Emmons, L., Apel, E., Pfister, G., van Donkelaar,

been observed reaching North America near the Canadianborder at Cheeka Peak (Jaffe et al., 1999).

Satellite observations offer a top-down constraint on emis-sions. Previous work includes absolute emissions of nitro-gen oxides (Leue et al., 2001; Martin et al., 2003a; Jaegléet al., 2005; M̈uller and Stavrakou, 2005), volatile organiccompounds (Palmer et al., 2003; Fu et al., 2007), and carbonmonoxide (Arellano et al., 2004; Heald et al., 2004; Pétron etal., 2004), as well as trends in nitrogen oxide (Richter et al.,2005; van der A et al., 2006; Zhang et al., 2007) emissions.The clearest signals in current SO2 retrievals are of volcanicactivity (Khokar et al., 2005), although anthropogenic ac-tivity has also been detected (Eisinger and Burrows, 1998;Krotkov et al., 2006; Carn et al., 2007). In some regionssatellite-retrieved Aerosol Optical Depth (AOD) is closelyrelated to SO2 emissions through production of SO=4 (Massieet al., 2004; Dubovik et al., 2008).

Springtime weather patterns generally produce thestrongest seasonal outflow from Asia (Jacob et al., 2003; Liuet al., 2005), and can result in a pronounced influence ofAsian emissions upon the North American continent. DuringApril and May 2006, the Intercontinental Chemical Trans-port Experiment, Phase B (INTEX-B) set out to assess thisinfluence using a combination of aircraft and satellite mea-surements throughout the northeastern Pacific (Singh et al.,20082). This NASA-driven initiative constituted the secondhalf of the INTEX project, and was designed to improve theunderstanding of gas and aerosol transformation and trans-port on transcontinental and intercontinental scales.

In this paper we investigate the long-range transport ofEast Asian SO=4 to Canada. Section 2 presents the aircraftcomponent of the Canadian contribution to INTEX-B andoutlines the other instruments, platforms and the model usedin this study. In Sect. 3, we estimate the recent growth in EastAsian SOx emissions based upon remote sensing measure-ments. Section 4 combines data from a chemical transportmodel with in-situ measurements to characterize the Asiansulfur transport to Canada during INTEX-B. This sectiongoes on to assess the development of East Asian SO=4 influxto North America between 1985 and INTEX-B using aircraftdata from both periods. A case study of an Asian plume ispresented in Sect. 5, along with the implications for Cana-dian air quality. Conclusions are in Sect. 6.

A., Millet, D., Heald, C., and Jimenez, J.-L.: Evolution of Asianaerosols during transpacific transport in INTEX-B, Atmos. Chem.Phys. Discuss., in preparation, 2008.

2Singh, H. B., Brune, W. H., Crawford, J. H., Jacob, D. J., Rus-sel, P. B., et al.: An overview of the INTEX-B campaign: Transportand transformation of pollutants over the Pacific and the Gulf ofMexico, Atmos. Chem. Phys. Discuss., in preparation, 2008.

Atmos. Chem. Phys., 8, 2999–3014, 2008 www.atmos-chem-phys.net/8/2999/2008/

A. van Donkelaar et al.: Long-range transport of East Asian sulfur to Canada 3001

2 INTEX-B platforms

Here we introduce the aircraft, surface and satellite measure-ments, and the model used for interpretation.

2.1 In-situ measurements

Figure 1 provides an overview of the measurement platformsand regions examined throughout this manuscript. Severalaircraft participated in INTEX-B, including the NASA DC-8, the NSF C-130 and a Canadian Cessna 207 described be-low. Throughout this manuscript, we limit the DC-8 and C-130 measurements to within the boxed regions of Fig. 1 tofocus on long-range transport of Asian aerosol to Canada.The DC-8 aircraft utilized both a mist chamber (Cofer etal., 1985) and bulk aerosol filters to characterize the SO=4aerosol load, during 10 flights between 17 April 2006 and15 May 2006. The size cutoff of the onboard mist cham-ber system is∼1µm (based on estimated particle transmis-sion efficiency through the inlet and sampler) while that ofthe bulk aerosol filters has been empirically determined tobe∼4.5µm (McNaughton et al., 2007). Mist chamber sam-pling periods are less than two minutes and aerosol filters arenot exposed longer than 10 to 20 min, depending upon al-titude. Uncertainties in the reported SO=4 mixing ratios are∼20% from the mist chamber and∼25 pptv (∼110 ng/m3)from the filters. A chemical ionization mass spectrometer(CIMS) instrument (Huey et al., 2004; Kim et al., 2007) wasalso onboard the DC-8 and used for the measurement of SO2with a sampling frequency of approximately 3 s.

The C-130 platform included a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) (DeCarloet al., 2006; Canagaratna et al., 2007; Dunlea et al., 20081)with ∼12 s sampling frequency and a particle-into-liquidsampler (PILS) (Weber et al., 2001; Peltier et al., 2008) ofone minute sampling frequency during its 11 flights between21 April 2006 and 15 May 2006. AMS particle transmis-sion is approximately PM1 in vacuum aerodynamic diame-ter (Jayne et al., 2000) with particle transmission efficiencyrapidly decreasing for aerosols larger than 0.7µm (e.g. Ru-pakheti et al., 2005; Liu et al., 2007). A collection effi-ciency (CE) of 0.5 is used for the AMS on the C-130 andis based on many previous intercomparisons (Canagaratna etal., 2007, and references therein), with a correction for in-creased CE under high acidity conditions (Quinn et al., 2006)as discussed by Dunlea et al. (2008)1. PILS measurementswere restricted to particles less than 1µm (at 1 atm. pres-sure) aerodynamic diameter via a single-stage micro-orificeimpactor (Model 100, MSP Corp.). AMS and PILS sulfatemeasurement uncertainties are estimated at 25% and 10%,respectively.

Whistler Peak Station (50.1◦ N, 122.9◦ W, 2182 m) is op-erated by Environment Canada and has provided continuousmeasurements of meteorological data, CO and O3 since itsestablishment in 2002 (Macdonald et al., 2006). Inorganic

filter packs of SO=4 , NO−

3 and Ca+ are also routinely col-

lected and analyzed. In addition to these regular measure-ments, a HR-ToF-AMS (Zhang et al., 20083) and a Micro-Orifice Uniform Deposit Impactor (MOUDI) were operatedat the site for the duration of INTEX-B. The MOUDI was op-erated with three stages to isolate particles into three nominalsize bins of3µm.

A Cessna 207 aircraft, supplied by Environment Canadaduring INTEX-B, contained a suite of instruments designedto capture both trace gases and aerosol pollutants (Leaitch etal., 20084). Aerosol instrumentation included number con-centrations of ultra-fine aerosol (PMS7610), aerosol size dis-tribution (FSSP300:


Fig. 2. Flight paths of the Cessna 207 aircraft during the INTEX-B campaign over 22 April 2006 to 17 May 2006. The left panelshows all Cessna 207 flights, with colors representing individualflights. The right panel highlights the 3 May 2006 inter-comparisonflight between the Cessna and C-130 aircraft. The flight track ofthe Cessna is shown in red, and of the C-130 in blue. The grey boxdefines the inter-comparison region.

time differences, we compare only measurements taken dur-ing the Cessna upward spiral against those from the C-130.

Figure 3 shows the speciated aerosol profiles from bothaircraft during this intercomparison. All measurements areconverted to concentrations at standard temperature and pres-sure of 1013 hPa and 0◦C. Significant agreement is found be-tween the AMS measurements, with respective Root MeanSquare Differences (RMSD) and mean bias of, 0.9 and0.3µg/m3 for SO=4 , 0.3 and 0.2µg/m

3 for organics, 0.03 and0.003µg/m3 for nitrate, and 0.2 and−0.0007µg/m3 for am-monium. The largest disagreement is found in SO=4 at ap-proximately 625 hPa, likely representative of a change in airmass, as indicated by significant and abnormal disagreement(∼30%) between the relative humidity measurements on thetwo aircraft. Measurements at this particular pressure weresampled∼35 minutes apart. Removal of points between 600and 650 hPa, decreases the RMSD and bias in SO=4 to 0.6 and−0.01µg/m3 respectively, leaving other species largely un-changed. This is considered good agreement for these sam-pling conditions.

MOUDI measurements of the SO=4 size distribution atWhistler Peak during INTEX-B indicate a mean ratio of totalSO=4 aerosol to SO

=

4 below 1µm in aerodynamic diameterof 1.4. This value is likely more appropriate for lower tropo-spheric SO=4 , which is the focus of this study, than for uppertropospheric SO=4 . We scale the submicron SO

=

4 measure-ments by this correction factor, which is further justified inSect. 4, to better represent total SO=4 mass. Airborne mea-surements off the west coast of Washington State and Ore-gon in May 1985 found that up to half of the non-seasaltSO=4 mass was above 1.5µm (Andreae et al., 1988) suggest-ing either the use of a larger scale factor may be appropriate,

Fig. 3. Aerosol Mass Spectrometer (AMS) measurements from theMay 3, 2006 inter-comparison flight over Whistler Peak Station(50.1◦ N, 122.9◦ W). Cessna data are shown in red. C-130 data areshown in blue. All data at STP. No scaling for the upper size cut ofthe AMS has been applied to these data.

or that a change in the SO=4 size distribution has occurredbetween these flight periods. MOUDI measurements of theNO−3 size distribution indicate that total NO

−

3 aerosol is eighttimes larger than submicron NO−3 . However, we do not applya correction factor to nitrate measurements due to concernsabout such a large scale factor.

Figure 4 shows average vertical profiles of Cessna Q-AMSand water (H2O) concentration data obtained during fourseparate enhancement periods. SO=4 concentrations of 1–3µg/m3 dominate in the free troposphere and tend to in-crease with altitude, implying long-range transport. In con-trast, organic concentrations typically decrease with altitudeand dominate at the surface, implying a local source. Theseopposing trends suggest that the amount of organics trans-ported with SO=4 is small and that long-range transport of or-ganic aerosols is not a significant contributor to the organicconcentration in the region studied. Leaitch et al. (2008)4

find a high level of mass closure with Cessna Q-AMS mea-surements, suggesting that the relatively high Q-AMS de-tection limit for organics (0.4–0.6µg/m3) has not impactedthis conclusion. They also show that the occurrence of in-creased sulfate usually accompanies an increase in the num-ber and mass concentrations of coarse particles. Dunlea etal. (2008)1 find that SO=4 concentrations exceed those of or-ganics for all Asian plume intercepts in the C-130, with olderair masses being characterized by a larger SO=4 /organics ra-tio than younger ones having undergone more rapid trans-port, presumably due to additional production of SO=4 dur-ing their extended transport time. The organic enhancementover 15 May–17 May is likely fuelled by an unusually highmixed layer depth, as indicated by the water concentration



profile, and can be attributed to local sources. This periodis further examined by Zhang et al. (2008)3 and McKendryet al. (2008). The contribution of nitrate to particulate massis relatively insignificant, in part reflecting AMS size restric-tions. We focus on long-range transport of SO=4 for the re-mainder of the manuscript.

2.2 Model Description

We use the GEOS-Chem chemical transport model v7-04-09 (Bey et al., 2001) (http://www-as.harvard.edu/chemistry/trop/geos/index.html) to interpret the aforementioned mea-surements. GEOS-Chem is driven by assimilated meteo-rological data from the Goddard Earth Observing System(GEOS-4) at the NASA Global Modeling Assimilation Of-fice (GMAO), with 30 vertical levels and degraded to themodel’s horizontal resolution of 2◦ latitude by 2.5◦ longi-tude.

The aerosol simulation in GEOS-Chem includes thesulfate-nitrate-ammonium system (Park et al., 2004; Park etal., 2006), carbonaceous aerosols (Park et al., 2003; Liaoet al., 2007), mineral dust (Fairlie et al., 2007) and sea-salt(Alexander et al., 2005). The aerosol and oxidant simula-tions are coupled through formation of sulfate and nitrate(Park et al., 2004), heterogeneous chemistry (Jacob, 2000)and aerosol effects on photolysis rates (Martin et al., 2003b).Wet and dry deposition are based upon Liu et al. (2001),including both washout and rainout. GEOS-Chem capturesboth the timing and distribution of Asian dust outbreaks dur-ing TRACE-P and ACE-Asia (Fairlie et al., 2007). It exhibitsno significant bias in Asian SOx (SO2+SO=4 ) outflow dur-ing spring 2001 as part of the TRACE-P campaign (Park etal., 2005), although modeled SO=4 concentrations were 50%high during ACE-Asia, which may suggest an error in SO2oxidation rates (Heald et al., 2005).

The global emission inventory in the standard GEOS-Chem model is based on GEIA (Benkovitz et al., 1996)for the base year of 1985 with scale factors to 1998.We implement here the EDGAR 3.2FT2000 emission in-ventory based upon the year 2000 (Olivier et al., 2001)to provide a more current estimate of global emissionsof NOx, SOx and CO. The global inventory is replacedby regional inventories from NEI99 (http://www.epa.gov/ttn/chief/net/1999inventory.html) over the United States for1999, BRAVO (Kuhns et al., 2005) over Mexico for 1999 andStreets et al. (2003, 2006) for 2000 (NOx and SOx) and 2001(CO) for eastern Asia. EMEP emissions (http://www.emep.int) of NOx and CO are used over Europe for up to 2000.We update the eastern Asia emissions to 2006 from Streetset al. (http://www.cgrer.uiowa.edu/EMISSIONDATA new/index 16.html and implement CAC emissions (http://www.ec.gc.ca/pdb/cac/) over Canada for 2005 and EMEP SOxemissions (Vestreng et al., 2007) over Europe for the year2004.

Fig. 4. Cessna Q-AMS vertical profiles of sulfate, organics and ni-trate during four enhancement periods. Sulfate (SO=4 ) data havebeen scaled by multiplying with a factor of 1.4 to account for par-ticle size restrictions as inferred from MOUDI measurements atWhistler summit. Aerosol data are at STP. Water (H2O) concen-tration profiles are in cyan. Date ranges are indicated in the bottomright of each plot. Error bars represent one standard deviation ofthe data. A small vertical offset is included between datasets forvisibility.

We scale all regional and global inventories from their re-spective base year to 2003, the last year of available statis-tics, unless its base year is after 2003. Our approach fol-lows Bey et al. (2001) and Park et al. (2004). Emissions arescaled according to estimates provided by individual coun-tries, where available. These countries/regions include theUnited States, Canada, Japan and Europe. NOx emissionsof remaining countries are scaled proportional to changes intotal CO2 emissions. SOx emissions are similarly scaled tosolid fuel CO2 emissions and CO emissions to liquid fuelCO2 emissions. A scale factor of 4.1% per year is used forship emissions (Corbett et al., 2007). CO2 emission dataare obtained from the Carbon Dioxide Information AnalysisCenter (CDIAC).

In addition to annual scale factors, diurnal scale factors arealso applied to NOx emissions. Here, the intra-day variationof each grid cell is based upon the diurnal variation of eachsource type, as provided with the EDGAR inventory, and itsrelative contribution to total NOx emissions within that cell.

www.atmos-chem-phys.net/8/2999/2008/ Atmos. Chem. Phys., 8, 2999–3014, 2008

http://www-as.harvard.edu/chemistry/trop/geos/index.htmlhttp://www-as.harvard.edu/chemistry/trop/geos/index.htmlhttp://www.epa.gov/ttn/chief/net/1999inventory.htmlhttp://www.epa.gov/ttn/chief/net/1999inventory.htmlhttp://www.emep.inthttp://www.emep.inthttp://www.cgrer.uiowa.edu/EMISSION_DATA_new/index_16.htmlhttp://www.cgrer.uiowa.edu/EMISSION_DATA_new/index_16.htmlhttp://www.ec.gc.ca/pdb/cac/http://www.ec.gc.ca/pdb/cac/


Fig. 5. Aerosol Optical Depth (AOD) from the MODIS and MISR satellite instruments and a GEOS-Chem simulation. The top row showsmean AOD over 2000–2006 and defines Region 1 as used in the lower panels. The middle panel shows monthly mean retrieved and simulatedAOD for Region 1 with simulated SOx emissions held at 2000 levels. Simulated contributions of dust and SO=4 to total AOD are also shown.Highlighted areas indicate time periods used in the lower panels. The bottom left panel shows the Region 1 difference between retrieved andsimulated AOD averaged between July and December of each year expressed as a percentage of mean retrieved AOD. Dashed line indicatesbest linear fit, error bars represent the 20th and 80th percentile. The bottom right panel shows the simulated relationship in Region 1 betweentotal AOD and SOx emissions over July–December 2000–2006 as calculated with 5 simulations with SOx emissions increased by 0%, 5%,10%, 15% and 20%. The red and blue stars respectively indicate the observed change in difference of simulated AOD between MISR andMODIS. Error bars denote one standard deviation of the data.

2.3 Satellite instrumentation

Aerosol Optical Depth (AOD), a measure of light extinction,has been retrieved since 2000 from the Moderate ResolutionImaging Spectroradiometer (MODIS) and Multi-angle Imag-ing Spectroradiometer (MISR), onboard the NASA satelliteTerra. The MODIS retrieval of AOD is based on scene

brightness over dark surfaces, using empirical relationshipsin the spectral variation in surface reflectivity (Kaufmann etal., 1997; Remer et al., 2005). We use the MODIS collec-tion 5 dataset (Levy et al., 2007). The MISR algorithm usesobserved differences in the spatial variation of backscatteredradiation with changing viewing angle to self-consistently re-trieve surface reflectivity and AOD (Martonchik et al., 2002;



Kahn et al., 2005). Global coverage in the absence of cloudsis achieved daily from MODIS and in 6 to 9 days from MISR.

3 Estimate of sulfur emission growth from China

Significant increases in AOD retrieved from the Total OzoneMapping Spectrometer (TOMS) over China between 1979–2000 and the Advanced Very High Resolution Radiome-ter (AVHRR) off the east coast of China between the peri-ods 1988–1991 and 2002–2005 are attributed to increasedaerosol sources (Massie et al., 2004; Mishchenko and Ge-ogdzhayer, 2007). Here we investigate recent retrievals ofAOD from MODIS and MISR and assess their relationshipwith Chinese sulfur emissions growth. We first use GEOS-Chem, with East Asian anthropogenic emissions held at year2000 levels, to investigate meteorologically induced changesto AOD.

The top row of Fig. 5 shows mean AOD for 2000–2006over East Asia from MODIS, MISR and GEOS-Chem. Sim-ulated AOD includes all major aerosol types (mineral dust,sulfate-nitrate-ammonium, carbonaceous, and sea-salt). Aregion of pronounced enhancement, designated as Region 1,is apparent in all three datasets. MODIS AOD exceedsMISR AOD by 12% over this region, consistent with com-parisons by Abdou et al. (2005). Simulated AOD exceedsMISR AOD by 22% and exhibits a smoother distributionthan both retrievals, with a more centralized maximum thatreflects the temporally static emissions used. The middlepanel of Fig. 5 presents monthly average AOD within theRegion 1. All three datasets contain a distinct seasonal vari-ation with a spring maximum and a fall minimum that re-flects the seasonal variation in dust as noted by Prospero etal. (2002). Simulated AOD generally captures the retrievedmonthly variation and magnitude as compared to both in-struments (MODIS:r2=0.46, RMSD=0.09; MISR:r2=0.36,RMSD=0.12), although the simulation tends to overestimatespringtime AOD. Simulated AOD contributions from dust(green) and SO=4 (magenta) indicate that dust comprises thelargest fraction of springtime AOD, whereas SO=4 dominatesduring other periods. We focus on the periods between Julyand December, as indicated by yellow bars, when an average56% of total AOD results from the presence of SO=4 , com-pared to 17% from dust.

The bottom left panel of Fig. 5 shows the annual meandifference over July–December between simulated and re-trieved AOD for Region 1, expressed as a percentage ofthe mean retrieved AOD from each instrument over thesix-year, low-dust period. We find a significant trendin the satellite-model AOD difference for both MODIS(+4.1%/year,r2=0.72) and MISR (+3.4%/year,r2= 0.54).We associate this trend with increased SOx emissions, asSO=4 dominates simulated AOD in this comparison, simu-lated SOx emissions are held at 2000 levels and interannualchanges of non-anthropogenic aerosols, such as dust and sea

salt, are accounted for in the simulation. Trends in otheraerosols could play a role, but would be less apparent due totheir smaller AOD over this region during July–December.

The quantitative relationship between AOD and SO2 emis-sions depends on a number of factors including SO2 oxi-dation rates, dynamics and aerosol deposition (Dubovik etal., 2008). We quantify the relationship by conducting sensi-tivity simulations with increased SOx emissions, and exam-ining the change in simulated AOD. The bottom right panelof Fig. 5 shows the calculated relationship between SOxemissions and AOD over Region 1. The calculated ratio of1AOD%/1SOx emissions % is nearly linear over this regionduring July to December. The annual trends in the differencebetween simulated and retrieved AOD correspond to simu-lations with an annual growth in SOx emissions of 6.2%/yrfor MISR and 9.6%/yr for MODIS. In general agreement, acomparison of the two bottom-up SOx emission inventoriesfor 2000 (Streets et al., 2003) and 2006 (http://www.cgrer.uiowa.edu/EMISSIONDATA new/index16.html) over Re-gion 1 yields an annual growth of 9.9%. Beyond actual emis-sion growth, changes between the 2000 and 2006 inventoriesinclude the addition of local inventories not present in, andimprovement and corrections made to, the original 2000 in-ventory. These factors may account for the slight discrepancybetween the growth estimates. We adopt the 2006 bottom-upinventory for our standard simulation, as it provides addi-tional information on the spatial distribution of these SOxemissions.

4 Campaign average analysis of transpacific transport

The top row of Fig. 6 shows campaign average SOx concen-trations for the DC-8 over the domain in Fig. 1. Filter packand mist chamber measurements of SO=4 have been com-bined with corresponding CIMS SO2 measurements. Bothfilter pack and mist chamber based measurements show amaximum around 700 hPa. Heald et al. (2006) attribute theSO=4 maximum in the lower free troposphere to preferentialscavenging during transport either in the boundary layer orduring lifting to the upper troposphere. Our standard sim-ulation of total SOx captures the relative vertical profile offilter pack based measurements over the domain of the DC-8, but overestimates their magnitude between 500–900 hPawith a RMSD of 0.32µg/m3 (mean bias=15%). Mist cham-ber SO=4 measurements are scaled by 1.4 to account forsupermicron aerosol as described in Sect. 2.1. Over 500–900 hPa, the campaign average filter pack measurements are33% higher than the unscaled mist chamber measurements,lending support to this scale factor. Mist chamber basedSOx measurements are well captured over the same range(RMSD=0.20µg/m3, mean bias=7.5%). Direct comparisonof filter pack and mist chamber SO=4 with simulated valuesshow weaker agreement (Filter Pack: RMSD=0.47µg/m3,mean bias=42%; Mist Chamber: RMSD=0.42µg/m3, mean


http://www.cgrer.uiowa.edu/EMISSION_DATA_new/index_16.htmlhttp://www.cgrer.uiowa.edu/EMISSION_DATA_new/index_16.html


Fig. 6. Campaign average aircraft measurements of SOx and SO=4 during INTEX-B, within the boundaries shown in Fig. 1. Simulated casesinclude our standard simulation, no East Asian emissions, and no global anthropogenic emissions. All measured and modeled data are atSTP. Mist Chamber, AMS and PILS SO=4 data are increased by a factor of 1.4 to account for particle size restrictions. Error bars denote onestandard deviation. A small vertical offset is included between datasets for visibility.

bias=42%) than for SOx, likely reflecting an overestimate inthe SO2 oxidation rate (Heald et al., 2005). However, thebias in SO=4 found here for the East Pacific is lower thanfound by Heald et al. (2005) for the West Pacific, suggestinga decrease with air mass age as continued SO=4 productionduring transport decreases the ratio of SO2 to SOx.

The bottom panels of Fig. 6 show campaign averageSO=4 measurements on the C-130 and Cessna, sampled co-incidently in time and space with simulated concentrations.Campaign average SO=4 concentrations for the C-130 mea-surements generally increase with altitude, reaching a maxi-mum at 600 hPa. The C-130 HR-ToF-AMS measurementsconsistently exceed the PILS measurements, indicative ofcurrent uncertainties in aerosol measurement technologies.During a blind intercomparison conducted 15 May 2006during a period of DC-8 and C-130 formation flying, theDC-8 Mist Chamber and C-130 PILS sulfate were in closeagreement (slope=1.00, 1 sigma=0.03µg/m3, range 0.15 to1.15µg/m3, r2=0.95). The C-130 had considerable freedomto chase individual events. Despite this, simulated total SO=4

between 500–900 hPa has an RMSD of 0.40µg/m3 (meanbias=34%) versus C-130 HR-ToF-AMS measurements andan RMSD of 0.54µg/m3 (mean bias=59%) versus C-130PILS measurements. The simulation exhibits the weak en-hancement at 600 hPa, although fails to represent the lowerconcentrations at lower altitudes.

The sampling strategy for the Cessna was to conductfrequent profiles over Whistler Peak. Such a samplingstrategy facilitated comparison with simulated results, pro-vided context for the measurements at Whistler summit,and accommodated the range and duration of the Cessna.Cessna measurements indicate a fairly uniform vertical pro-file, with a large standard deviation in the free tropospherethat reflects an oscillation between clean conditions andplumes. The simulation agrees significantly with size-correction scaled measured SO=4 (RMSD=0.13µg/m

3, meanbias=2.5%). While recognizing the potential influence ofboth measurement uncertainty and the limitation of apply-ing a constant size-correction factor across both altitude andaircrafts, the eastward decrease in the bias between the DC-



8 and Cessna aircraft may indicate an increasing SO=4 /SOxratio in the measurements.

Figure 6 also shows simulations without anthropogenicEast Asian and all anthropogenic sources for all three air-craft flight tracks. Anthropogenic East Asian SOx dominatesthroughout the DC-8 profiles, comprising 60% of the sim-ulated mass between 500–900 hPa, with the largest contri-bution in the lower free troposphere. Other anthropogenicSOx sources comprise an additional 17%. For the C-130flight track, closer to North America, the sensitivity simu-lation attributes 67% of SO=4 to be of Asian origin, with apeak at 600 hPa. For the Cessna profiles over Whistler, localsources are most significant below 850 hPa, with the influ-ence of East Asian anthropogenic emissions increasing withaltitude. We calculate that 56% of the measured SO=4 be-tween 500–900 hPa is from East Asia. Model analysis indi-cates the influence of East Asian sources at higher altitudes inboth C-130 and Cessna versus the DC-8 measurements. Thisorographic effect is induced by rising air masses on approachto North American mountain ranges.

Of interest is the evolution of Asian sulfate over the lasttwo decades. Figure 7 shows the mean non-seasalt sulfateprofile observed by Andreae et al. (1988) during 4 flightsin May 1985 using the NCAR King Air, covering a part ofthe C-130 INTEX-B flight domain. SO=4 concentrations (ad-justed to STP at 273 K, sum of coarse and fine fractions) in-creased with altitude below 5 km, from 0.3–0.6µg/m3 in themarine boundary layer to 0.6–0.8µg/m3 in the cloud con-vection layer and free troposphere. The 1985 measurementsthus showed lower concentrations, but a similar trend withincreased altitude as was seen in the C-130 measurements.Mean C-130 measurements between 500–900 hPa are higherthan the 1985 data by 60% from the PILS and by 90% fromHR-ToF-AMS.

Differences in measurement techniques, flight tracks andmeteorology could contribute to the apparent trend. There-fore we further interpret these observations by conducting aGEOS-Chem simulation using 1985 emissions and meteorol-ogy and sampling along the 1985 flights tracks. Global emis-sions for 1985 are taken from GEIA (Bentovitz et al., 1996),except for East Asia which are based on Streets et al. (2003,2006) and scaled following Streets et al. (2000b, 2006). Thesimulation reproduces the measurements with an RMSD of0.25µg/m3 (mean bias=21%) over 500–900 hPa. A sensitiv-ity simulation without anthropogenic East Asian emissionsreveals that this source contributes 0.14µg/m3 (20%) to themeasured values in 1985, significantly reduced compared tothe 67% along the C-130 flights in 2006.

To account for meteorological variation between 1985 and2006, we also simulate the 2006 INTEX-B period using1985 emissions. The relative contribution of East AsianSO=4 to the C-130 area (April–May, 34–55

◦ N, 123.75–141.25◦ W, 500–900 hPa) between 1985 and 2006 increased72% under identical meteorological conditions. The rel-ative contribution in the King Air (April–May, 45-49◦ N,

Fig. 7. Average aircraft measurements of SO=4 during 1985 KingAir flights, within the boundary shown in Fig. 1. Simulated casesof total SO=4 include our standard simulation, no East Asian emis-sions, and no global anthropogenic emissions. All measured andmodeled data are at STP. Error bars denote one standard deviation.

123.75–126.25◦ W, 500–900 hPa) and Cessna (April–May,49–51◦ N, 123.75–121.23◦ W, 500–900 hPa) flight regionsincrease similarly by 74% and 85%, respectively.

5 Asian plume development and influence

Figure 8 examines the development of an Asian plume from18 April 2006 to 25 April 2006. MODIS AOD retrievalsfrom both the Aqua (1:30 overpass) and Terra (10:30 over-pass) satellites are plotted with simulation results from thesame period. The GEOS-Chem simulation successfully cap-tures many of the features associated with the influx event,which is dominated by dust, and also carries SO=4 . Both re-trieval and simulation show this plume beginning from Chinaon 18 April and stretching across the Pacific Ocean through21 April, and finally sweeping down from the north whilemoving eastward over the North American coast. This eventis further discussed by McKendry et al. (2008).

Figure 9 shows individual Cessna and GEOS-Chem SO=4profiles taken between 22 and 25 April, during the arrivalof this Asian plume. The accuracy of individual simulatedprofiles, shown in the left panel cluster, varies with RMSDranging between 0.39–0.87µg/m3. Simulations can fail toproduce accurate plumes (e.g. Dunlea et al., 20081), butin this case the simulated plume has been transported tooquickly, with simulated concentrations exceeding measure-ments on 24 April, but the opposite on 25 April. During



Fig. 8. The development of an Asian plume between 18–25 April 2006 as retrieved from MODIS and as simulated by GEOS-Chem. Whitespaces indicate regions of less than 10 cloud free scenes within a 2◦×2.5◦ area.

long range transport events, small errors in the meteorologi-cal fields used by chemical transport models can compoundto create offsets in time and space, making individual modelprofiles less representative than average comparisons. Theright panel shows a mean profile comparison during this pe-riod. Significant agreement (RMSD=0.25µg/m3) suggeststhis event was well represented, despite the weaker agree-ment of individual profiles.

Figure 10 shows simulated average conditions duringApril and May 2006. The top panel shows mean concen-trations at 2 km, where DC-8 SO=4 enhancements were ob-served. Simulated SO=4 along the North American Pacificcoast show increased concentrations relative to western con-tinental regions. Major regional anthropogenic sources pro-duce a large increase in SO=4 concentrations over easternUnited States and Canada. The middle and bottom panelsshow vertical cross-sections of SO=4 and percentage of SO

=

4originating from East Asia, respectively, averaged betweenthe blue lines of the top panel. The highest overall magni-tude (>1µg/m3) is again simulated in eastern North Amer-ica and is predominately from regional emissions. Nonethe-

less, a narrow band of Asian influence in excess of 40% pre-vails over the continent at 4.5 km, where overall concentra-tions are∼0.3µg/m3. Along coastal regions, the largest EastAsian influence is found between 1 and 5 km, where 40% ofthe overall SO=4 burden originated in East Asia. Interactionwith the planetary boundary layer is facilitated by a combina-tion of plume subsidence and mountain-induced mixing pro-cesses typical of southern British Columbia (McKendry etal., 2001). We calculate that surface concentrations of SO=4along the southern Pacific Canadian coast are increased by0.31µg/m3 (∼30%) as a result of Asian emissions in spring.We take the mean model bias as compared to the C-130 andCessna aircraft to estimate an error of approximately 25% inthis calculation. Heald et al. (2006) found a 0.16µg/m3 en-hancement in SO=4 over the northwest United States duringperiods of Asian influence. Yu et al. (2008) used MODISobservations to access the seasonal variation in transpacificpollution aerosol and conclude that springtime transport isabout twice as large as during other seasons.



Fig. 9. Cessna Q-AMS SO=4 profiles taken 22–25 April 2006. The left-hand panels show individual flight profiles. The right panel shows amean profile of the same data. Error bars represent one standard deviation of the data. Q-AMS data are at STP and scaled a factor of 1.4 toaccount for particle size restrictions. A small vertical offset is included between the datasets for visibility.

We go on to explore the surface SO=4 measurements fromthe National Air Pollution Surveillance (NAPS) Network inthe Vancouver area for evidence of Asian influence. Fig-ure 11 shows surface SO=4 concentrations between April andMay 2006 in the Vancouver area as a function of the mod-eled percent SO=4 originating in Asia. The two measurementsites in the Vancouver area, Abbotsford and Vancouver, re-side in the same model grid box. Black circles correspond tomeasurement averages, binned at intervals of 5% simulatedEast Asian influence. Individual measurements show sub-stantial scatter, but linear regression of the binned measure-ments show a significant correlation (r2=0.82). Binned mea-surements indicate that an additional 0.32µg/m3 reaches thesurface with each 10% increase in modeled Asian SO=4 , cor-roborating that current levels of Asian sulfur emissions areimpacting surface SO=4 concentrations in Canada. Aerosoltransport events are episodic and the daily influence of EastAsian SO=4 varies dramatically. Figure 11 suggests thatduring plumes East Asian SO=4 can contribute more than1.5µg/m3 to coastal western Canadian concentrations. Thisis of similar magnitude to enhancements observed by theCessna Q-AMS during plume events shown in Fig. 4.

6 Conclusions

We interpreted a suite of satellite (MODIS and MISR), air-craft (DC-8, C-130 and Cessna 207) and ground-based mea-surements (Whistler Peak, NAPS) over the North PacificOcean and western North America in April–May 2006 aspart of the INTEX-B campaign to understand the implica-tions of long-range transport of Asian aerosol to Canada.

The Canadian component of INTEX-B included 33 flightsfrom a Cessna 207 aircraft. We compare the Cessnaquadrupole Aerodyne Mass Spectrometer (Q-AMS) mea-surements with a high resolution time of flight AMS(HR-ToF-AMS) onboard the C-130 during an intercom-parison flight, yielding an overall bias of−0.01µg/m3

with appreciable scatter (RMSD=0.6µg/m3) for sulfate(SO=4 ) and similar agreement for organics (bias=0.2µg/m

3,RMSD=0.3µg/m3). However, there was a small systematicdifference (


Fig. 10. Average simulated conditions for April and May 2006.The top panel shows total SO=4 concentrations at∼2 km altitude.Middle and bottom panels display the mean cross-sectional totalconcentration and East Asian influence, respectively, between theblue lines in the top panel.

show SO=4 enhancements of 1–2µg/m3 over 600–700 hPa,

indicative of long-range transport, whereas organic enhance-ments are largest near the surface, suggesting a local emis-sion source. We did not detect long-range transport of sig-nificant organic aerosol from the Cessna data, contrary to ex-pectations.

We interpret these observations with a global chemicaltransport model, GEOS-Chem, to simulate the implicationsof anthropogenic activity. We implement a more recentglobal bottom-up inventory (EDGAR) and develop updatedscale factors, bringing global anthropogenic emissions from1998 to at least the year 2003. We also implement currentbottom-up inventories for East Asia for 2006, Canada for2005 and Europe for 2004.

Retrieved Aerosol Optical Depth (AOD) from MISR andMODIS during low dust periods (Jul–Dec) are used to eval-uate the growth of SOx emissions between 2000–2006. Wefind a growth in the difference between simulated and re-trieved AOD of 3.4%/yr (MISR) and 4.1%/yr (MODIS) us-ing constant anthropogenic emissions sources as representedby GEOS-Chem. GEOS-Chem calculations of the change inAOD for a change in SOx emissions indicate a near-linear

Fig. 11. The influence of Asian SO=4 on coastal western Canadiansurface concentrations during April-May 2006. Black circles de-note mean filter pack sulfate measurements from Canada’s NationalAir Pollution Network sites in Vancouver and Abbotsford as aver-aged at 5% intervals of percent Asian SO=4 . Dashed line showslinear best fit. Percent Asian sulfate is simulated using the GEOS-Chem model.

relationship over East Asia. We estimate the average annualgrowth in East Asian SOx emissions to be between 6.2% us-ing MISR and 9.6% using MODIS, supporting the bottom-upestimate of an annual increase of 9.9% from 2000 (Streets etal., 2003) to 2006.

We use this simulation to understand the characteristics ofEast Asian outflow as measured by the INTEX-B aircraft.Over the Pacific Ocean, Asian outflow of SOx is strongestin the lower troposphere, with enhanced SO=4 concentrationsof 1–1.5µg/m3 observed by the DC-8 between 700–800 hPa.The mean C-130 and Cessna aircraft SO=4 measurements of1–1.5µg/m3 over 600–800 hPa indicate that Asian plumesare often elevated by orographic effects along coastal NorthAmerica. The simulation generally captures the campaignmean profile shape of DC-8 SOx, and C-130/Cessna SO=4measurements, with RMSD of 0.13–0.54µg/m3 (mean biasof 2.5–59%). Bias in simulated SO2 oxidation likely con-tribute to the lower agreement found with respect to the C-130 measurements. Simulations without Asian emissions re-veal that long-range transport of SO=4 dominates campaign-mean aircraft measurements in the free troposphere.



We compare the INTEX-B measurements with aircraftmeasurements in May 1985 over a similar domain as theC-130. Measured free tropospheric SO=4 concentrations in-crease by 60–90% from 1985 to 2006. Sensitivity simula-tions for 1985 and without East Asian emissions indicatethat their relative contribution to SO=4 concentrations duringApril and May between 500–900 hPa increased by 72–85%due to emission changes as compared to 1985 depending onthe specific region.

Comparison of individual plumes with aircraft profiles andMODIS AOD reveals a general consistency, but offsets intime and space. Campaign-mean simulations show that 50%of the SO=4 burden between 1 and 5 km over Whistler is ofanthropogenic Asian origin. These emissions increase sur-face concentrations along the western Canadian coast by anaverage 0.31µg/m3 (∼30%) in spring. This effect is corrob-orated with surface measurements, where we find an increaseof 0.32µg/m3 with each 10% increase in simulated fractionof Asian SO=4 during INTEX-B, with episodic enhancementsof more than 1.5µg/m3.

A better understanding of SO2 oxidation is still needed.Previous work (Heald et al., 2006) and our analysis indicatean overestimate in the simulated SO2 oxidation rate. Devel-opment of size-resolved aerosol simulations and SO=4 instru-ments that sample larger particles with high time resolutionwould facilitate model-measurement comparison. Improvedunderstanding of inter-instrument SO=4 measurements wouldbe valuable.

Acknowledgements.We thank David Parrish for his helpfulsuggestions. Rob Buchanan provided professional and tirelesspiloting of the Cessna during the study. Tragically, Rob lost hislife in the crash of the Cessna following the conclusion of thestudy. Mohammed Wasey, Armand Gaudenzi, Dave Halpin andJohn Deary provided technical and logistical support. Specialthanks to Juniper Buller, Anton Horvath, the Whistler Ski Patroland Whistler Blackcomb for their support. This work is supportedby the Natural Sciences and Engineering Research Council ofCanada Special Research Opportunity Program and EnvironmentCanada. Edward J. Dunlea and Jose L. Jimenez were supported byNASA grants NNG04GA67G and NNG06GB03G and NSF grantsATM-0449815 and ATM-0513116. Rodney Weber was fundedthrough NASA grant NNG06GA68G. We thank the MODIS andMISR teams for their level 3 aerosol products and EnvironmentCanada for their NAPS data.

Edited by: H. Singh

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